A particle beam apparatus conventionally comprises an optical element such as an objective lens. The optical element, in particular an objective lens, usually suffers from imperfections so that its desired idealized functionality may not be provided. An objective lens may have, for example, an astigmatism deteriorating optical properties so that the focal length of the objective lens is dependent of the position at which a beam is incident onto the objective lens.
In a particle beam system such as a particle beam microscope, an astigmatism of an objective lens may be corrected for by utilizing a stigmator. The stigmator functions analog to a cylinder lens, i.e. a beam incident on the stigmator is spread along a principle axis of the stigmator and focused in a direction perpendicular to the principle axis and the direction of the beam.
Conventionally, a stigmator in a particle beam apparatus provides a quadrupole field acting analog to a cylinder lens. However, the quadrupole field must be properly aligned to the particle beam in order to properly compensate the astigmatism of the objective lens. If not properly aligned, a particle beam of the particle beam apparatus traversing the stigmator undergoes an unwanted deflection. Furthermore, a variation of, for example, an excitation of the stigmator results in an undesired movement of an acquired image.
Conventionally, the adjusting of the stigmator to the particle beam is performed manually by varying the configuration of the stigmator until image movements become minimum when periodically changing the strength of the stigmator excitation by a small amount. Manually adjusting the stigmator is elaborate and time consuming. In addition, the user manually adjusting the stigmator must be experienced in order to achieve an appropriate adjustment. If the quadrupole field of the stigmator is not properly aligned to the optical axis of the particle beam system, in addition, the adjusting of the stigmator leads to a movement of the image and makes the adjustment of the stigmator even more laborious.
The present invention has been made taking the above considerations into account.